Our feathered friends provide us with some beautiful examples of the link between brain and behaviour. In some bird species, groups of cells involved in seasonal behaviours die after they have performed their function, but are regenerated by neurogenesis as and when they are needed.

Male songbirds, for example, serenade females; the brain nuclei which produce the vocalizations die when the mating season ends, and regenerate as the next one approaches. Similarly, the Clarke’s nutcracker stores tens of thousands of pine seeds in many different caches spread across a wide area; afterwards, cells in the areas of the brain involved in spatial memory die, but regenerate later in the year, so that the caches can be retrieved.

Two years ago, veteran bird brain researcher Fernando Nottebohm of Rockefeller University, who was involved in this work, published a study which provided another link between behaviour and brain structure. He and his colleagues showed that the survival of newborn neurons in the forebrain of the adult zebra finch (Taeniopygia guttata) was significantly affected by the complexity of the environment.

Nottebohm’s latest study, which has just been published in the Journal of Neuroscience, builds on these findings. It shows how the turnover of newly generated cells in the zebra finch is choreographed to meet the demands of changes in social context. Moreover, a study due to be published next month suggests that these findings can be extrapolated to humans.

Nottebohm, together with lead author Einat Adar of Tel Aviv University, and Anat Barnea of the Open University of Israel, reared male zebra finches in three outdoor breeding colonies. When the birds reached 45-60 days of age, each was removed from the colony and transferred to an outdoor cage, which contained 3 other zebra finches. At 4 months of age, the birds were injected with a chemical called 5-bromo-2′-deoxyuridine (BrdU), which becomes incorporated into newly-synthesized DNA, and can therefore be used to detect dividing cells in living tissues.

After a further 30 or 90 days in their cages, the zebra finches were moved again, into one of two social settings. One group of birds was transferred to a “simple” environment, an aviary containing just one unfamiliar adult female zebra finch; the others were placed in a “complex” environment, containing a group of 40-45 adults of both sexes.

40 days after being exposed to their new environments, both groups of birds were killed by an overdose of anaesthetic, and their brains removed for analysis. The brains were dissected, and the numbers of BrdU+ (that is, newborn) cells per cubic millimeter in two different regions of the brain was estimated. One of these regions, the nidopallium caudale (NC), is involved in the production of birdsong. The other, the hippocampal complex (HC), is involved in spatial memory, and is particularly large in those bird species that cache food.

The researchers found that the number of dividing cells was significantly affected not only by the nature of the social change, but also by the time at which that change took place. They also observed that the effects of the environment on neurogenesis were dependent upon the position of the new cells within the brain.

Increased social complexity promoted the survival of 1-month-old cells, in both of the regions examined. This was accompanied by increased death of 3-month-old cells. On the other hand, reduced social complexity promoted the survival of the slightly older cells, but also to increases in the death of 1-month-old cells. Finally, it was found that the complex social setting promoted the survival of younger new cells located towards the back of the NC more than of the cells towards the front.

One interpretation of these findings is that younger cells are not yet committed to a specific function, and have not yet formed fully functional connections. They are therefore more able than older cells to cope with the cognitive demands of the complex new environment, and are recruited to process and store the social information. Older neurons have already established their connections, and are committed to a certain job; they cannot encode large amounts of new information, and so become redundant when the bird encounters a complex new social setting. They therefore die off because of disuse, and are replaced by younger cells which can cope with the demands of the new environment.

With regard to the effect of the location of the cells, the authors suggest that this may be because those new cells located toward the back of the NC are more able to respond to new information with their existing connections, or can modify their connectivity to encode the new information.

So how might all this be relevant to humans? We know that neurogenesis occurs in the olfactory bulb and hippocampus of the adult human brain. Some studies show that suppressing neurogenesis prevents the formation of olfactory memories in insects. And although it has been suggested that newborn neurons in the adult brain are also involved in memory formation, there is no evidence for this.

A new study, to be published in next month’s issue of The American Journal of Public Health, provides a tentative link between the zebra finch and humans. The study is described in this article from the New York Times:

Researchers at the Harvard School of Public Health used data gathered from 1998 to 2004 from the Health and Retirement Study, a large, nationally representative population of American adults ages 50 and older. Participants took memory tests at two-year intervals during the study period. Testers read a list of 10 common nouns to survey respondents, who were then asked to recall as many words as possible immediately and again after a five-minute delay. The researchers also measured social integration based on marital status, volunteer activities, and contact with parents, children and neighbors.

The results showed that individuals who in their 50s and 60s engaged in a lot of social activity also had the slowest rate of memory decline. In fact, compared to those who were the least socially active, study subjects who had the highest social integration scores had less than half the rate of memory loss. The researchers controlled for variables like age, gender, race and health status. Those who had the fewest years of formal education appeared to have the most to gain from an active social life as they aged. The study showed that the protective effect of social integration was greatest among individuals with fewer than 12 years of education.

In light of this, it is tempting to speculate that maintaining an active social life promotes the survival of neurons in the hippocampus, neurons which are involved in encoding social information and memories, and which would be prone to dying in the absence of social interactions. As it is now well established that physical exercise induces neurogenesis in the hippocampus, one might therefore suggest that a life-long regime of regularly meeting old friends and making new ones, combined with regular exercise, is an effective way of reducing age-related cognitive decline.

Similarly, the Clarke’s nutcracker stores tens of thousands of pine seeds in many different caches spread across a wide area; afterwards, cells in the areas of the brain involved in spatial memory die, but regenerate later in the year, so that the caches can be retrieved.

If these cells die without ‘erasing’ the memory of the cache locations, then these memories must be stored elsewhere. Is it known where?

The birds do not remember all the places, what they do is associate good hiding places and hence go searching there. They do not remember each location, they simply remember what sort of locations that are good places to hide nuts.